Medical image processing apparatus and x-ray computed tomography apparatus

ABSTRACT

A medical image processing apparatus according to an embodiment includes a separating unit, a reconstructing unit, and an extracting unit. The separating unit separates projection data into pieces of line-integrated data each of which corresponds to a different one of basis materials set in advance. The reconstructing unit reconstructs pieces of basis material image data from the pieces of line-integrated data each of which corresponds to a different one of the basis materials, the pieces of basis material image data being configured so that each pixel value of each of pixels indicates an abundance ratio of corresponding each of the basis materials that is present at each of the pixel. The extracting unit extracts an artifact region, on a basis of attenuation coefficients of each of the pixels calculated from the pieces of basis material image data each of which corresponds to a different one of the basis materials.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/JP2013/072771, filed on Aug. 26, 2013 which claims the benefit ofpriority of the prior Japanese Patent Application No. 2012-190237, filedon Aug. 30, 2012 and Japanese Patent Application No. 2013-174909, filedon Aug. 26, 2013, the entire contents of which are incorporated hereinby reference.

FIELD

Embodiments described herein relate generally to a medical imageprocessing apparatus and an X-ray computed tomography apparatus.

BACKGROUND

Conventionally, a method for obtaining images is known by which imagetaking processing is performed by an X-ray Computed Tomography (CT)apparatus while using multi different levels of X-ray tube voltages.When two mutually-different levels of X-ray tube voltages are used, themethod may be called a “dual-energy CT” method. In “dual-energy CT”, twoprojection data obtained from two mutually-different levels of X-raytube voltages are separated into projection data (line-integrated data)each corresponding to respective predetermined two basis materials, andfrom each of the two separated data, image based on the abundance ratioof the basis materials (basis material image) is reconstructed, theabove-described which applied technology is known. According to suchapplied technology, it is possible to obtain various types of imagessuch as a monochromatic X-ray image, a density image, an effectiveatomic number image, and the like, by performing a weighted calculationwhile using the two basis material images.

The applied technology described above is effective in correctingartifacts caused by beam hardening. However, besides the artifactscaused by beam hardening, there are other various artifacts such asthose caused by a degradation in the precision level of the projectiondata due to highly-absorbent materials and those caused by scatteredrays.

In particular, artifacts often occur due to a degradation in theprecision level of the projection data caused by highly-absorbentmaterials. The reason can be explained as follows: When a material(e.g., metal) having a large linear absorption coefficient is present inan image taking target, the count of a detector shows a very small valueduring an image taking processing using a low level of X-ray tubevoltage, and it is therefore not possible to obtain proper projectiondata. In that situation, it is not possible to properly obtain theprojection data of the basis materials. As a result, the acquiredmonochromatic X-ray image will have an artifact where, for example,information in the surroundings of the highly-absorbent material ismissing. According to the applied technology described above, it is notpossible to generate a monochromatic X-ray image from which the impactsof the artifacts other than the artifacts caused by beam hardening arealso eliminated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an exemplary overall configuration of an X-ray CTapparatus according to a first embodiment;

FIG. 2 is a diagram of exemplary configurations of a pre-processing unitand an image generating unit according to the first embodiment;

FIG. 3 is a chart for explaining an extracting unit according to thefirst embodiment;

FIG. 4 is a chart of an example of processing results obtained by acorrecting unit according to the first embodiment;

FIG. 5 is a drawing of a contour of processing according to the firstembodiment;

FIG. 6 is a flowchart of an exemplary processing performed by the X-rayCT apparatus according to the first embodiment;

FIG. 7A and FIG. 7B are drawings for explaining a fourth embodiment.

DETAILED DESCRIPTION

A medical image processing apparatus according to an embodiment includesa separating unit, a reconstructing unit, and an extracting unit. Theseparating unit separates projection data into pieces of line-integrateddata each of which corresponds to a different one of a plurality ofbasis materials that are set in advance. The reconstructing unitreconstructs pieces of basis material image data from the pieces ofline-integrated data each of which corresponds to a different one of theplurality of basis materials. the pieces of basis material image databeing configured so that each pixel value of each of pixels indicates anabundance ratio of corresponding each of the basis materials that ispresent at each of the pixel. The extracting unit extracts an artifactregion, on a basis of attenuation coefficients of each of the pixelscalculated from the pieces of basis material image data each of whichcorresponds to a different one of the plurality of basis materials.

Exemplary embodiments of a medical image processing apparatus will beexplained in detail below, with reference to the accompanying drawings.In the following sections, an X-ray computed tomography (CT) apparatusthat functions as the medical image processing apparatus will beexplained in the exemplary embodiments.

First Embodiment

First, an exemplary overall configuration of an X-ray CT apparatusaccording to a first embodiment will be explained, with reference toFIG. 1. FIG. 1 is a diagram of the exemplary overall configuration ofthe X-ray CT apparatus according to the first embodiment. As shown inFIG. 1, the X-ray CT apparatus according to the first embodimentincludes a gantry device 10, a couch device 20, and a console device 30.

The gantry device 10 is a device configured to radiate X-rays onto anexamined subject (hereinafter, a “subject”) P and to acquire X-raydetection data and includes a high voltage generating unit 11, an X-raytube 12, an X-ray detector 13, a data acquiring unit 14, a rotatingframe 15, and a gantry driving unit 16.

The high voltage generating unit 11 is a device configured to generate ahigh voltage and to supply the generated high voltage to the X-ray tube12. The X-ray tube 12 is a vacuum tube that generates X-rays with thehigh voltage supplied from the high voltage generating unit 11. TheX-rays generated by the X-ray tube 12 are radiated onto the subject P.

The X-ray detector 13 is a detector that detects the X-ray detectiondata indicating an intensity distribution of the X-rays that wereradiated from the X-ray tube 12 and have passed through the subject P.In other words, the X-ray detector 13 detects the X-ray detection dataindicating a degree of X-ray absorption occurring inside the body of thesubject P. For example, the X-ray detector 13 is a two-dimensional arraydetector in which a plurality of rows of detecting elements are arrangedalong the body axis direction of the subject P (i.e., the Z-axisdirection shown in FIG. 1), each row of detecting elements being made upof a plurality of X-ray detecting elements that are arranged along thechannel direction (i.e., the Y-axis direction shown in FIG. 1).

The rotating frame 15 supports the X-ray tube 12 and the X-ray detector13 in such a manner that the X-ray tube 12 and the X-ray detector 13oppose each other while the subject P is interposed therebetween. Thegantry driving unit 16 is a driving device that causes the X-ray tube 12and the X-ray detector 13 to revolve on a circular orbit centered aboutthe subject P, by driving the rotating frame 15 to rotate.

The data acquiring unit 14 is a Data Acquisition System (DAS) andacquires the X-ray detection data detected by the X-ray detector 13.More specifically, the data acquiring unit 14 acquires the X-raydetection data corresponding to each of the directions (hereinafter,“X-ray radiation directions”) in which the X-rays are radiated from theX-ray tube 12. The X-ray radiation directions may be referred to as“views”. Furthermore, the data acquiring unit 14 performs an amplifyingprocessing and/or an Analog/Digital (A/D) conversion processing on theacquired X-ray detection data corresponding to each of the views andoutputs the result to a pre-processing unit 34 (explained later)included in the console device 30. For example, the data acquiring unit14 outputs data (sinogram data) obtained by arranging the X-raydetection data in a time-series manner for each of the X-ray radiationdirections, the X-ray detection data indicating an X-ray detectionamount for each of the X-ray detecting elements.

The couch device 20 is a device on which the subject P is placed. Asshown in FIG. 1, the couch device 20 includes a couchtop 22 and a couchdriving device 21. The couchtop 22 is a bed on which the subject P isplaced. The couch driving device 21 moves the subject P into the insideof the rotating frame 15, by moving the couchtop 22 along the body axisdirection of the subject P (i.e., the Z-axis direction).

The console device 30 is a device that receives operations performed onthe X-ray CT apparatus by an operator and to reconstruct a tomographyimage from a group of data acquired by the gantry device 10. As shown inFIG. 1, the console device 30 includes an input device 31, a displaydevice 32, a scan controlling unit 33, the pre-processing unit 34, aprojection data storage unit 35, an image generating unit 36, an imagestorage unit 37, and a system controlling unit 38.

The input device 31 includes a mouse, a keyboard, a button, a trackball,a joystick, and/or the like that are used for inputting various types ofinstructions by the operator such as a medical doctor or a technicianwho operates the X-ray CT apparatus and transfers various types ofcommands received from the operator to the system controlling unit 38(explained later).

The display device 32 includes a monitor that displays a Graphical UserInterface (GUI) used for receiving an instruction from the operator viathe input device 31 and to display images stored in the image storageunit 37 (explained later).

The scan controlling unit 33 controls operations of the high voltagegenerating unit 11, the gantry driving unit 16, the data acquiring unit14, and the couch driving device 21. Thus, the scan controlling unit 33controls an X-ray scanning processing performed on the subject P by thegantry device 10, as well as an acquiring processing of a group of X-raydetection data and a data processing performed on the group of X-raydetection data.

More specifically, the scan controlling unit 33 causes an X-ray scan tobe performed by causing X-rays to be radiated continuously orintermittently from the X-ray tube 12, while causing the rotating frame15 to rotate. For example, the scan controlling unit 33 causes a helicalscan to be performed so that images are taken by causing the rotatingframe 15 to rotate continuously while the couchtop 22 is being moved orcauses a conventional scan to be performed so that images are taken bycausing the rotating frame 15 to rotate with a single complete cycle orcontinuously, while the position of the subject P is fixed.

The pre-processing unit 34 generates projection data by performing alogarithmic conversion processing and a correcting processing such as anoffset correcting processing or a sensitivity correcting processing, onthe X-ray detection data transmitted from the data acquiring unit 14.Processing performed by the pre-processing unit 34 according to thefirst embodiment will be explained in detail later.

The projection data storage unit 35 stores therein the projection datagenerated by the pre-processing unit 34.

The image generating unit 36 generates various types of images from theprojection data stored in the projection data storage unit 35 and tostore the generated images into the image storage unit 37. For example,the image generating unit 36 reconstructs an X-ray CT image byperforming a back-projection processing (e.g., a back-projectionprocessing realized by implementing a Filtered Back Projection (FBP)method) on the projection data and stores the reconstructed X-ray CTimage into the image storage unit 37. Processing performed by the imagegenerating unit 36 according to the first embodiment will be explainedin detail later.

The system controlling unit 38 exercises overall control of the X-ray CTapparatus, by controlling operations of the gantry device 10, the couchdevice 20, and the console device 30. More specifically, by controllingthe scan controlling unit 33, the system controlling unit 38 controlsthe acquiring processing of the group of X-ray detection data performedby the gantry device 10 and the couch device 20. Furthermore, bycontrolling the pre-processing unit 34 and the image generating unit 36,the system controlling unit 38 controls image processing performed bythe console device 30. Furthermore, the system controlling unit 38exercises control so that the various types of images stored in theimage storage unit 37 are displayed on the display device 32.

The overall configuration of the X-ray CT apparatus according to thefirst embodiment has thus been explained. In addition to acquiringprojection data by performing an image taking processing while the X-raytube voltage is fixed to one level, the X-ray CT apparatus according tothe first embodiment configured as described above also acquiresprojection data by performing a “multi-energy image taking processing”while using multi different levels of X-ray tube voltages. For example,the X-ray CT apparatus according to the first embodiment acquires theprojection data by performing a “dual-energy image taking processing”while using two mutually-different levels of X-ray tube voltages.

The “dual-energy image taking processing” may be performed, for example,by implementing any of the following three image taking methods. A firstimage taking method is called a “slow-kV switching method (two-rotationmethod)” by which an image taking processing is performed at first byusing a first X-ray tube voltage, and subsequently, an image takingprocessing is performed by using a second X-ray tube voltage. A secondimage taking method is called a “dual source method (two-tube method)”by which an image taking processing is performed while usingmutually-different X-ray tube voltages, with the use of a two-tube X-rayCT apparatus, instead of the one-tube X-ray CT apparatus shown inFIG. 1. A third image taking method is called a “fast-kV switchingmethod (a high-speed switching method)” by which an image takingprocessing is performed by switching X-ray tube voltages at a high speedin correspondence with each of the views, while causing the rotatingframe 15 to rotate. By using any of these methods, it is possible toobtain two types of raw data (projection data) having mutually-differentenergy levels.

In the following sections, an example will be explained in which the“dual-energy image taking processing” is performed by implementing thehigh-speed switching method. The first embodiment is also applicable tosituations where the “dual-energy image taking processing” is performedby implementing the two-rotation method or the two-tube method.

In recent years, applied technology has been developed by which twopieces of projection data obtained by using two mutually-differentlevels of X-ray tube voltages are separated into pieces of projectiondata (line-integrated data) each of which corresponds to a different oneof two basis materials that are set in advance, so that an image(hereinafter, a “basis material image”) based on the abundance ratio ofeach of the two basis materials is reconstructed. According to thisapplied technology, it is possible to obtain various types of imagessuch as a monochromatic X-ray image, a density image, an effectiveatomic number image, and the like, by performing a weighted calculationwhile using the two basis material images.

The applied technology described above is effective in correctingartifacts caused by beam hardening. For example, it is possible togenerate an X-ray CT image of monochromatic X-rays (a monochromaticX-ray image or a monochromatic X-ray CT image) in which the impact ofbeam hardening is reduced compared to that in a conventional X-ray CTimage of continuous X-rays. However, besides the artifacts caused bybeam hardening, there are other various artifacts such as those causedby a degradation in the precision level of the projection data due tohighly-absorbent materials and those caused by scattered rays.

According to the applied technology described above, it is not possibleto generate a monochromatic X-ray image from which the impacts of theartifacts other than the artifacts caused by beam hardening are alsoeliminated. To cope with this situation, according to the firstembodiment, processing described below are performed by thepre-processing unit 34 and the image generating unit 36, so as toextract artifacts occurring in a monochromatic-X-ray image.

FIG. 2 is a diagram of exemplary configurations of the pre-processingunit and the image generating unit according to the first embodiment. Asshown in FIG. 2, the pre-processing unit 34 according to the firstembodiment includes a projection data generating unit 34 a and aseparating unit 34 b. Furthermore, as shown in FIG. 2, the imagegenerating unit 36 according to the first embodiment includes areconstructing unit 36 a, an extracting unit 36 b, and a correcting unit36 c.

The projection data generating unit 34 a generates the projection databy performing a logarithmic conversion processing or the like on theX-ray detection data transmitted from the data acquiring unit 14. In thefirst embodiment, the projection data generating unit 34 a generatesprojection data (hereinafter, “high energy projection data”) from X-raydetection data obtained by using a first X-ray tube voltage (e.g., 130kV). Furthermore, in the first embodiment, the projection datagenerating unit 34 a generates projection data (hereinafter, “low energyprojection data”) from X-ray detection data obtained by using a secondX-ray tube voltage (e.g., 80 kV).

The separating unit 34 b separates the projection data into pieces ofline-integrated data each of which corresponds to a different one of aplurality of basis materials (i.e., two or more basis materials) thatare set in advance. In the first embodiment, the projection data isrepresented by two pieces of projection data (the high energy projectiondata and the low energy projection data) acquired by using twomutually-different levels of X-ray tube voltages.

Furthermore, in the first embodiment, the plurality of basis materialsare two basis materials, which are, for example, bones and water. In thefollowing sections, one of the two basis materials will be referred toas a first basis material, whereas the other will be referred to as asecond basis material.

More specifically, the separating unit 34 b separates the high energyprojection data and the low energy projection data into line-integrateddata (first line-integrated data) of the first basis material andline-integrated data (second line-integrated data) of the second basismaterial. In this situation, the basis materials are specified out ofmaterials of which the mass attenuation coefficients at various levelsof energy are known.

The first line-integrated data and the second line-integrated dataseparated by the separating unit 34 b are stored into the projectiondata storage unit 35.

Furthermore, the reconstructing unit 36 a reconstructs pieces of basismaterial image data from the pieces of line-integrated data each ofwhich corresponds to a different one of the plurality of basismaterials, the pieces of basis material image data being configured sothat each pixel value of each of pixels (or the voxels) indicates anabundance ratio of corresponding each of the basis materials that ispresent at each of the pixel. More specifically, by performing aback-projection processing on the first line-integrated data, thereconstructing unit 36 a reconstructs basis material image data(hereinafter, “first basis material image data”) of the first basismaterial. Furthermore, by performing a back-projection processing on thesecond line-integrated data, the reconstructing unit 36 a reconstructsbasis material image data (hereinafter, “second basis material imagedata”) of the second basis material. In this situation, the pixel valueof a pixel “i” in the first basis material image data indicates theabundance ratio “c₁” of the first basis material at the pixel “i”.Similarly, the pixel value of a pixel “i” in the second basis materialimage data indicates the abundance ratio “c₂” of the second basismaterial at the pixel “i”.

In this situation, the attenuation coefficient “μ(E)” in an image takingsite corresponding to the pixel “i” at an arbitrary level of energy “E”can be calculated by using Expression (1) shown below. In Expression (1)below, “μ₁(E)” denotes the attenuation coefficient for the first basismaterial at “E”, whereas “μ₂(E)” denotes the attenuation coefficient forthe second basis material at “E”.

μ(E)=c ₁μ₁(E)+c ₂μ₂(E)  (1)

The CT value “CT#(E)” of the image taking site corresponding to thepixel “i” at “E” can be calculated by assigning the value “μ(E)”calculated from Expression (1) and an attenuation coefficient “μ(E)” ofwater at “E” to Expression (2) shown below.

$\begin{matrix}{{{CT}\# (E)} = {1000 \times \frac{{\mu (E)} - {\mu_{w}(E)}}{\mu_{w}(E)}}} & (2)\end{matrix}$

The reconstructing unit 36 a is thus able to generate a monochromaticX-ray image at the arbitrary level of energy “E”, by using the basismaterial image data and Expressions (1) and (2). In the attenuationcoefficient calculated from Expression (1), the error caused by beamhardening is reduced. However, the attenuation coefficient calculatedfrom Expression (1) still has, for example, impacts of metal artifacts,artifacts caused by bones and a contrast agent, and cone beam artifacts.

To cope with this situation, the extracting unit 36 b shown in FIG. 2extracts an artifact region, on a basis of the attenuation coefficientsof each of the pixels calculated from the pieces of basis material imagedata each of which corresponds to a different one of the plurality ofbasis materials. The extracting unit 36 b according to the firstembodiment extracts the artifact region by comparing attenuationcoefficients at two mutually-different energy levels with each other,within an energy range that does not include absorption edge energies.FIG. 3 is a chart for explaining the extracting unit according to thefirst embodiment.

By using Expression (1), the extracting unit 36 b according to the firstembodiment calculates an attenuation coefficient for each of the pixelsat each of the two energy levels (E₁ and E₂). As for the magnituderelationship between the two levels of energy, “E₁<E₂” is satisfied. Inthis situation, the mass attenuation coefficient (the linear attenuationcoefficient/density) of each of the materials exhibits a formation asshown in FIG. 3, in relation to the photon energy. In FIG. 3, the massattenuation coefficient of water is indicated with a solid line, whereasthe mass attenuation coefficient of bones (cortical bones) is indicatedwith a broken line, while the mass attenuation coefficient of iodine isindicated with a dot-and-dash line.

As shown in FIG. 3, in energy regions where the photoelectric effect orthe Compton scattering is dominant, “μ(E₁)>μ(E₂)” is satisfied for anymaterial, except in the region near the absorption edge energies wherethe attenuation coefficient is discontinuous. On a basis of this fact,the extracting unit 36 b determines that correct values of “c₁” and “c₂”are not obtained at such a pixel where “μ(E₁)>μ(E₂)” is not satisfiedand thus extracts such a pixel as an artifact region. In other words, ifthe magnitude relationship between the attenuation coefficients at thetwo mutually-different energy levels calculated from “c₁” and “c₂” of agiven pixel exhibits a physical contradiction, the extracting unit 36 bdetermines that the pixel is in an artifact region.

The two levels of energy can be set by the operator within a range thatexcludes the absorption edge energies of each of the two basis materialsthat have been set. Alternatively, the two levels of energy may beconfigured into the apparatus in advance as an initial setting, inaccordance with the pair made up of the basis materials. Alternatively,the two levels of energy may be set by the extracting unit 36 b inaccordance with the pair made up of the basis materials.

After that, the correcting unit 36 c shown in FIG. 2 corrects theattenuation coefficients of the artifact region. For example, thecorrecting unit 36 c corrects the attenuation coefficients of theartifact region by utilizing the following notion: the values “c₁” and“c₂” in the artifact region are incorrect; however, among theattenuation coefficients at various levels of energy calculated from theincorrect values of “c₁” and “c₂”, an attenuation coefficient obtainedat a certain level of energy “E_cor” has a correct value.

In the range from which the absorption edge energies are excluded, thevalue of a correct attenuation coefficient becomes smaller, as the levelof energy becomes higher. In other words, a chart of a correctattenuation coefficient has a formation that falls toward the right. Incontrast, the attenuation coefficient of the artifact region extractedin the first embodiment becomes larger as the level of energy becomeshigher, even in the region excluding the absorption edge energies. Inother words, a chart of the attenuation coefficient of the artifactregion has a formation that rises toward the right. The chart of thecorrect attenuation coefficient having the formation that falls towardthe right intersects, at a certain point, the chart of the attenuationcoefficient of the artifact region having the formation that risestoward the right. The level of energy at the intersecting point servesas “E_cor” mentioned above.

Thus, the correcting unit 36 c obtains an energy level “E_cor” at whicheach of the attenuation coefficients of the artifact region calculatedfrom pixel values in the pieces of basis material image data each ofwhich corresponds to a different one of the plurality of basis materialsexhibits a substantially correct value. In the first embodiment, thecorrecting unit 36 c obtains the level of energy “E_cor” at which eachof the attenuation coefficients of the artifact region calculated fromthe pixel values in each of the first and the second basis materialimage data exhibits a substantially correct value. Furthermore, thecorrecting unit 36 c performs a correcting processing by using each ofthe attenuation coefficients of the artifact region at “E_cor” and theattenuation coefficient of predetermined materials at “E_cor”.

For example, as “E_cor”, the correcting unit 36 c obtains an energyvalue that is empirically or experimentally calculated in advance. Inthat situation, for example, the value of “E_cor” is configured into thesystem controlling unit 38 as an initial setting, so that the correctingunit 36 c obtains “E_cor” from the system controlling unit 38.Alternatively, the value of “E_cor” may be set by the operator via theinput device 31, when the correcting unit 36 c performs the processing.In that situation, the correcting unit 36 c obtains “E_cor” that hasbeen set through the input device 31, via the system controlling unit38.

The level of energy “E_cor” at which it is possible to obtain a correctattenuation coefficient is not necessarily always the same. For thisreason, in the first embodiment, a correlation between the attenuationcoefficient “μ(A)” at a certain level of energy “A” and “E_cor” iscalculated in advance. In this situation, “A” denotes a level of energythat is set in advance in accordance with the pair made up of the basismaterials, within the range that excludes the absorption edge energies.“μ(A)” is a value that can be calculated by assigning “c₁” and “c₂”, theattenuation coefficient of the first basis material at “A” and theattenuation coefficient of the second basis material at “A” toExpression (1).

As an example, the first embodiment utilizes the fact that “E_cor” and“μ(A)” satisfy a relationship of a linear function such as Expression(3) shown below that is expressed with a slope “a” and a Y-intercept“b”.

E_cor=a×μ(A)+b  (3)

In this situation, “A”, “a”, and “b” are values that can be obtainedexperimentally. Furthermore, in the first embodiment, the example isexplained in which the relationship between “E_cor” and “μ(A)” isexpressed with the linear function; however, the relationship between“E_cor” and “μ(A)” may be expressed with any of other various functionssuch as a polynomial function, an exponential function, or a logarithmicfunction. Furthermore, as mentioned above, when the correctingprocessing is performed while the value of “E_cor” is set to a constantvalue, it means that Expression (3) is set so as to satisfy “a=0;b=E_cor”.

The correcting unit 36 c calculates “E_cor” by assigning “μ(A)”calculated from “c₁” and “c₂” of the pixel extracted as an artifactregion to Expression (3). After that, the correcting unit 36 ccalculates an attenuation coefficient “μ(E_cor)” at “E_cor”, byassigning the attenuation coefficient of the first basis material, theattenuation coefficient of the second basis material, and “c₁” and “c₂”at “E_cor” to Expression (1). The attenuation coefficient “μ(E_cor)” isthe attenuation coefficient of the artifact region at “E_cor” and can beused as a value that approximates the true attenuation coefficient ofthe artifact region at “E_cor”.

After that, the correcting unit 36 c performs the correcting processingby using the attenuation coefficient “μ(E_cor)” and the attenuationcoefficient of a predetermined material at “E_cor”. For example, thepredetermined material may be water. In that situation, the correctingunit 36 c assigns an attenuation coefficient “μ_(w)(E)” of water at theenergy level “E” corresponding to a monochromatic X-ray image, anattenuation coefficient “μ_(w)(E_cor)” of water at “E_cor”, and“μ(E_cor)” to Expression (4) shown below. As a result, the correctingunit 36 c is able to obtain an attenuation coefficient “μ′(E)” after thecorrection (hereinafter, “corrected attenuation coefficient”) of thepixel extracted as an artifact region.

$\begin{matrix}{{\mu^{\prime}(E)} = {{\mu ({E\_ cor})} \times \frac{\mu_{w}(E)}{\mu_{w}({E\_ cor})}}} & (4)\end{matrix}$

In Expression (4) above, the attenuation coefficient of water is used;however, the first embodiment may be configured so that a correctedattenuation coefficient is calculated by using an attenuationcoefficient of any other appropriate material. In this situation, evenif the correcting processing is performed by setting “E_cor” to aconstant value, it is possible to obtain a corrected attenuationcoefficient of the artifact region by using Expression (4) above.

FIG. 4 is a chart of an example of processing results obtained by thecorrecting unit according to the first embodiment. In FIG. 4, a chart ofan attenuation coefficient (a linear attenuation coefficient) before thecorrection (hereinafter, a “pre-correction attenuation coefficient”) ofthe artifact region is indicated with a solid line, whereas a chart ofan attenuation coefficient (a linear attenuation coefficient) after thecorrection (a “corrected attenuation coefficient”) of the artifactregion is indicated with a broken line. As a result of the correctingprocessing by the correcting unit 36 c, the chart of the pre-correctionattenuation coefficient, which has a formation that rises toward theright, changes to the chart having a formation that falls toward theright, which exhibits no physical contradiction, as shown in FIG. 4.

The reconstructing unit 36 a shown in FIG. 2 generates a monochromaticX-ray image by using the corrected attenuation coefficients. Morespecifically, for the artifact region, the reconstructing unit 36 acalculates CT values by assigning the corrected attenuation coefficientsto Expression (2). Furthermore, for the region other than the artifactregion, the reconstructing unit 36 a calculates CT values by calculatingan attenuation coefficient from each of the pixel values in the firstbasis material image data and the second basis material image data byusing Expression (1) and further assigning the calculated attenuationcoefficient to Expression (2). Thus, the reconstructing unit 36 agenerates the monochromatic X-ray image at the energy level “E”.

After that, under the control of the system controlling unit 38, thedisplay device 32 displays the monochromatic X-ray image at the energylevel “E”.

FIG. 5 is a drawing of a contour of processing according to the firstembodiment. As shown in the top part of FIG. 5, when the correctingprocessing by the correcting unit 36 c has not been performed, a whiteartifact and a black artifact occur in the monochromatic X-ray image dueto a degradation in the precision level of the projection data caused byhighly-absorbent materials.

As shown in the middle part of FIG. 5, all of such a region is extractedas an artifact region, as a result of the extracting processing by theextracting unit 36 b. After that, as shown in the bottom part of FIG. 5,as a result of the correcting processing by the correcting unit 36 c, amonochromatic X-ray image is generated in which the artifact regioncaused by the highly-absorbent materials has been corrected.

When the correcting processing is performed while the value of “E_cor”is set to a constant value, it is acceptable for the operator to changethe value of “E_cor”. For example, the operator may refer to themonochromatic X-ray image generated by using the corrected attenuationcoefficient of the artifact region at the energy level “E” and maychange the value of “E_cor” if the operator has determined that thecorrection of the monochromatic X-ray image was not properly performed.In that situation, the correcting unit 36 c performs the correctingprocessing on the attenuation coefficient again by using the value“E_cor” that has been changed, so that the reconstructing unit 36 agenerates a monochromatic X-ray image by using the attenuationcoefficient that has been corrected again. Furthermore, when thecorrecting processing is performed while the value of “E_cor” is set toa constant value, it is also acceptable to set a plurality of valueseach as “E_cor”, so as to perform a correcting processing by using eachof the plurality of values and to generate a plurality of monochromaticX-ray images. In that situation, the operator is able to, for example,select a monochromatic X-ray image in which the artifacts have properlybeen reduced, from among the plurality of monochromatic X-ray images.Furthermore, even in the situation where “E_cor” is obtained by usingExpression (3) and where the attenuation coefficient of the artifactregion is corrected by using “E_cor” obtained from Expression (3) aswell as Expression (4), the operator may perform the processing ofchanging the value of “E_cor”.

Furthermore, the first embodiment may be configured so that a correctingprocessing is performed through any of the following processing in whichExpression (3) and (4) are not used. For example, the correcting unit 36c may perform a correcting processing by replacing each of theattenuation coefficients of the artifact region with attenuationcoefficients of predetermined materials. In that situation, thecorrecting unit 36 c performs the correcting processing by replacingeach of the attenuation coefficients of the artifact region with anattenuation coefficient of an arbitrary material. The arbitrary materialmay be, for example, a soft tissue.

Alternatively, the correcting unit 36 c may correct each of theattenuation coefficients of the artifact region by applying a correctingprocessing on the projection data or the line-integrated data thatpasses through the artifact region. For example, the correcting unit 36c corrects the high energy projection data and the low energy projectiondata that pass through the artifact region, so that the separating unit34 b is caused to separate, again, the corrected projection data intofirst line-integrated data and second line-integrated data.Alternatively, for example, the correcting unit 36 c corrects the firstline-integrated data and the second line-integrated data that passthrough the artifact region. After that, the correcting unit 36 c causesthe reconstructing unit 36 a to reconstruct basis material image dataagain from the first line-integrated data and the second line-integrateddata. As a result, the correcting unit 36 c is able to obtain acorrected attenuation coefficient.

Next, an exemplary processing performed by the X-ray CT apparatusaccording to the first embodiment will be explained, with reference toFIG. 6. FIG. 6 is a flowchart of the exemplary processing performed bythe X-ray CT apparatus according to the first embodiment.

As shown in FIG. 6, the separating unit 34 b included in the X-ray CTapparatus according to the first embodiment separates high energyprojection data and low energy projection data that have been acquiredinto first line-integrated data and second line-integrated data (stepS101). After that, the reconstructing unit 36 a reconstructs first basismaterial image data and second basis material image data from the firstline-integrated data and the second line-integrated data, respectively(step S102).

Subsequently, the extracting unit 36 b extracts an artifact region on abasis of the attenuation coefficient of each of the pixels calculatedfrom the first basis material image data and the second basis materialimage data (step S103), and the correcting unit 36 c corrects theattenuation coefficients of the artifact region (step S104).

After that, the reconstructing unit 36 a generates a monochromatic X-rayimage by using the corrected attenuation coefficients (step S105).Subsequently, the display device 32 displays the monochromatic X-rayimage (step S106), and the processing is ended.

As explained above, according to the first embodiment, such a pixel ofwhich the value of the attenuation coefficient calculated from the basismaterial image data exhibits a physical contradiction is extracted asthe artifact region. As a result, according to the first embodiment, itis possible to extract the artifacts occurring in the monochromaticX-ray image. Furthermore, according to the first embodiment, theattenuation coefficients of the artifact region are corrected, so thatthe monochromatic X-ray image is generated by using the correctedattenuation coefficients. In other words, according to the firstembodiment, it is possible to correct the weighting coefficients usedfor generating the monochromatic X-ray image. As a result, according tothe first embodiment, it is possible to reduce the artifacts in themonochromatic X-ray image.

Second Embodiment

As a second embodiment, another embodiment related to methods forextracting an artifact region implemented by the extracting unit 36 bwill be explained. The artifact region extracting method explained inthe first embodiment will be referred to as a first extracting method,whereas the artifact region extracting methods implemented by theextracting unit 36 b according to the second embodiment will bedescribed while being roughly divided into second to sixth extractingmethods.

The second extracting method utilizes the notion that it is physicallyimpossible for an attenuation coefficient to be 0 or smaller. Accordingto the second extracting method, the extracting unit 36 b extracts suchpixels of which the attenuation coefficients are 0 or smaller as theartifact region. The energy range used for the extraction is set on abasis of the first basis material and the second basis material thathave been set and within a range of X-ray tube voltages which the X-rayCT apparatus is capable of applying. In the following sections, theexemplary embodiment is explained in a case the energy range is set as“E_(a) to E_(b)”.

For example, the extracting unit 36 b sequentially calculatesattenuation coefficients within the range of “E_(a) to E_(b)”, on abasis of “c₁” and “c₂” of each of the pixels and Expression (1). Afterthat, the extracting unit 36 b extracts such a pixel that has a set madeup of “c₁” and “c₂” from which an attenuation coefficient of 0 orsmaller is calculated within the range of “E_(a) to E_(b)”, as anartifact region. According to this method, however, it is necessary tocalculate all the attenuation coefficients within the range of “E_(a) toE_(b)”. Thus, to reduce the load of the extracting processing, in thesecond extracting method, in a case a set made up of “c₁” and “c₂” atone pixel satisfies either of the three conditions, the pixel isextracted as an artifact region: The first condition is that both of thevalues of “c₁” and “c₂” are 0 or smaller;

The second condition is that the value “c₁” is a negative value, andalso Expression (5) shown below is satisfied where “R” denotes themaximum value of “μ₂(E)/μ₁(E)” within the range of “E_(a) to E_(b)”;

|c ₁ |≧R×|c ₂|  (5)

The third condition is that the value “c₂” is a negative value, and alsoExpression (6) shown below is satisfied where “R′” denotes the maximumvalue of “μ₁(E)/μ₂(E)” within the range of “E_(a) to E_(b)”.

|c ₂ |≧R′×|c ₁|  (6)

The values of “c₁” and “c₂” are known for each of all the pixels, andalso, “R” and “R′” are known. Thus, according to the second extractingmethod, it is possible to reduce the load of the extracting processingby making a judgment using the first to the third conditions.

Next, the third extracting method and the fourth extracting method willbe explained. According to the third and the fourth extracting methods,the extracting unit 36 b extracts the artifact region by comparing eachof the attenuation coefficients at a predetermined energy level withattenuation coefficients of materials that are set in advance at thepredetermined energy level. In this situation, the predetermined energylevel will be expressed as “E′”. The value “E′” may be set by theoperator or may be determined in an initial setting.

According to the third extracting method, a material (a “maximumabsorption material”) of which the absorption of X-rays that can bepresent in a human body is at the maximum serves as the materialdescribed above that is set in advance. According to the thirdextracting method, the extracting unit 36 b extracts such a pixel ofwhich the set made up of “c₁” and “c₂” satisfies the followingcondition: an attenuation coefficient “μ(E′)” at “E′” calculated byusing Expression (1) is larger than the attenuation coefficient of themaximum absorption material at “E′”, as an artifact region.

According to the fourth extracting method, a material (a “minimumabsorption material”) of which the absorption of X-rays that can bepresent in a human body is at the minimum serves as the materialdescribed above that is set in advance. According to the fourthextracting method, the extracting unit 36 b extracts such a pixel ofwhich the set made up of “c₁” and “c₂” satisfies the followingcondition: an attenuation coefficient “μ(E′)” at “E′” calculated byusing Expression (1) is smaller than the attenuation coefficient of theminimum absorption material at “E′”, as an artifact region.

Next, the fifth extracting method and the sixth extracting method willbe explained. According to the fifth and the sixth extracting methods,the extracting unit 36 b extracts the artifact region by comparing aratio between attenuation coefficients at two mutually-different energylevels with a ratio between attenuation coefficients of a material thatis set in advance at the two mutually-different energy levels. In thissituation, the two mutually-different energy levels will be referred toas “E₃ and E₄, where E₃<E₄”. The values of “E₃ and E₄” may be set by theoperator or may be determined in an initial setting.

According to the fifth extracting method, the maximum absorptionmaterial serves as the material described above that is set in advance.According to the fifth extracting method, the extracting unit 36 bcalculates a ratio “μ(E₃)/μ(E₄)” between the attenuation coefficient“μ(E₃)” at “E₃” and the attenuation coefficient “μ(E₄)” at “E₄” that arecalculated by using Expression (1). Furthermore, the extracting unit 36b obtains a ratio “μ_(a)(E₃)/μ_(a)(E₄)” between the attenuationcoefficient “μ_(a)(E₃)” at “E₃” and the attenuation coefficient“μ_(a)(E₄)” at “E₄” of the maximum absorption material. After that, theextracting unit 36 b extracts such a pixel of which the set made up of“c ₁” and “c₂” satisfies the following condition: the ratio“μ(E₃)/μ(E₄)” is larger than the ratio “μ_(a)(E₃)/μa(E₄)”, as anartifact region.

According to the sixth extracting method, the minimum absorptionmaterial serves as the material described above that is set in advance.According to the sixth extracting method, the extracting unit 36 bcalculates a ratio “μ(E₃)/μ(E₄)” between the attenuation coefficient“μ(E₃)” at “E₃” and the attenuation coefficient “μ(E₄)” at “E₄” that arecalculated by using Expression (1). Furthermore, the extracting unit 36b obtains a ratio “μ_(b)(E₃)/μ_(b)(E₄)” between the attenuationcoefficient “μ_(b)(E₃)” at “E₃” and the attenuation coefficient“μ_(b)(E₄)” at “E₄” of the minimum absorption material. After that, theextracting unit 36 b extracts such a pixel of which the set made up of“c₁” and “c₂” satisfies the following condition: the ratio “μ(E₃)/μ(E₄)”is smaller than the ratio “μ_(b)(E₃)μ_(b)(E₄)_(”), as an artifactregion.

Each of the first to the sixth extracting methods may be implementedsolely. Alternatively, two or more of these extracting methods may beimplemented in combination. By implementing two or more of the first tothe sixth extracting methods in combination, it is possible to improvethe level of precision of the artifact region extracting process. Thecorrecting processing and the monochromatic X-ray image generatingprocessing described in the first embodiment are also performed afterthe artifact region is extracted by using any of the methods describedin the second embodiment.

Third Embodiment

As a third embodiment, a method for further improving the level ofprecision of the artifact region extracting processing performed by theextracting unit 36 b will be explained.

When implementing any of the first to the sixth extracting methods, theextracting unit 36 b according to the third embodiment ensures that, asa seventh extracting method, such a pixel of which pixel values in amutually same position in the pieces of basis material image data eachof which corresponds to a different one of the plurality of basismaterials fall in a predetermined range is excluded from a target to beextracted as the artifact region. For example, the extracting unit 36 bdetermines that such a pixel of which the values of “c₁” and “c₂” areboth in the range of “0±α” should be excluded from the target of theextraction. In this situation, the value “α” may be set by the operatoror may be determined in an initial setting.

According to the third embodiment, as a result of implementing theseventh extracting method, it is possible to avoid the situation whereregions of air are extracted as noise.

Furthermore, when implementing any of the first to the sixth extractingmethods, the extracting unit 36 b according to the third embodiment mayimplement an eighth extracting method described below either togetherwith or without the seventh extracting method.

When implementing the eighth extracting method, the extracting unit 36 baccording to the third embodiment extracts the artifact region from dataobtained after a filtering processing is performed on the pieces ofbasis material image data each of which corresponds to a different oneof the plurality of basis materials. For example, after a filteringprocessing such as one that uses a median filter is performed on thepieces of basis material image data, the extracting unit 36 b extractsan artifact region. In this situation, the filtering processing may beperformed by a processing unit other than the extracting unit 36 b.

According to the third embodiment, by implementing the eighth extractingmethod, it is possible to eliminate isolated points caused by noise inthe basis material image data.

Fourth Embodiment

As a fourth embodiment, an example in which the artifact regionextracted by the extracting unit 36 b is indicated to a viewer will beexplained with reference to FIGS. 7A and 7B. FIGS. 7A and 7B aredrawings for explaining the fourth embodiment.

In the fourth embodiment, the system controlling unit 38 exercisescontrol so that the artifact region is displayed while being emphasizedwithin the monochromatic X-ray image using the corrected attenuationcoefficients. For example, under the control of the system controllingunit 38, the reconstructing unit 36 a renders, with a broken line, acontour of the artifact region within the monochromatic X-ray imageusing the corrected attenuation coefficients, as shown in FIG. 7A. Afterthat, the display device 32 displays an image as shown in FIG. 7A underthe control of the system controlling unit 38.

Alternatively, in the fourth embodiment, the system controlling unit 38exercises control so that the artifact region is displayed while beingemphasized within a monochromatic X-ray image using the attenuationcoefficients before the correction. For example, under the control ofthe system controlling unit 38, the reconstructing unit 36 a renders,with a broken line, a contour of the artifact region within themonochromatic X-ray image using the pre-correction attenuationcoefficients, as shown in FIG. 7B. After that, the display device 32displays an image as shown in FIG. 7B under the control of the systemcontrolling unit 38.

In this situation, the system controlling unit 38 may exercise controlso that a monochromatic X-ray image in which the extracted artifactregion is colored is displayed.

Also, in the fourth embodiment, when the artifact region is displayedwhile being emphasized within the monochromatic X-ray image using thepre-correction attenuation coefficients, the attenuation coefficientcorrecting processing by the correcting unit 36 c may be omitted.

In the fourth embodiment, because the artifact region within themonochromatic X-ray image is visualized, it is possible to present, forexample, any region that has a possibility of missing information due tohighly-absorbent materials, to an interpreting doctor who interprets themonochromatic X-ray image.

The medical image processing methods explained above in the first to thefourth embodiments are also applicable to a situation where a“multi-energy image taking processing” is performed while using three ormore mutually-different levels of X-ray tube voltages. Furthermore, themedical image processing methods explained above in the first to thefourth embodiments are also applicable to a situation where three ormore basis materials are set.

In the first to the fourth embodiments described above, the examples areexplained in which the X-ray detector 13 is an integral-type detector.However, the medical image processing methods explained above in thefirst to the fourth embodiments are also applicable to a situation wherethe X-ray detector 13 is a photon-counting-type detector thatindividually counts the light originated from the X-rays that havepassed through the subject P. When the X-ray detector 13 is a detectorof a photon-counting-type, the separating unit 34 b is able to calculatea linear attenuation coefficient from projection data acquired byperforming an image taking processing while the X-ray tube voltage isfixed to one level.

Furthermore, the medical image processing methods described above in thefirst to the fourth embodiments may be implemented by another medicalimage processing apparatus that is separately installed in addition tothe X-ray CT apparatus. In that situation, the medical image processingapparatus receives the projection data acquired by the X-ray CTapparatus and implements any of the medical image processing methodsdescribed above.

Furthermore, the constituent elements of the apparatuses and the devicesthat are shown in the drawings are based on functional concepts. Thus,it is not necessary to physically configure the elements as indicated inthe drawings. In other words, the specific mode of distribution andintegration of the apparatuses and the devices is not limited to theones shown in the drawings. It is acceptable to functionally orphysically distribute or integrate all or a part of the apparatuses andthe devices in any arbitrary units, depending on various loads and thestatus of use. Furthermore, all or an arbitrary part of the processingfunctions performed by the apparatuses and the devices may be realizedby a Central Processing Unit (CPU) and a computer program that isanalyzed and executed by the CPU or may be realized as hardware usingwired logic.

As explained above, according to at least one aspect of the first to thefourth embodiments, it is possible to extract the artifacts occurring inthe monochromatic X-ray image.

While certain embodiments of the present invention have been described,these embodiments have been presented by way of examples only, and arenot intended to limit the scope of the inventions. These exemplaryembodiments may be embodied in a variety of other forms; furthermore,various omissions, substitutions, and changes may be made withoutdeparting from the spirit of the inventions. The inventions defined inthe accompanying claims and their equivalents are intended to covervarious embodiments and modifications, in the same manner as thoseembodiments and modifications would fall within the scope and spirit ofthe inventions.

What is claimed is:
 1. A medical image processing apparatus comprising:a separating unit that separates projection data into pieces ofline-integrated data each of which corresponds to a different one of aplurality of basis materials that are set in advance; a reconstructingunit that reconstructs pieces of basis material image data from thepieces of line-integrated data each of which corresponds to a differentone of the plurality of basis materials, the pieces of basis materialimage data being configured so that each pixel value of each of pixelsindicates an abundance ratio of corresponding each of the basismaterials that is present at each of the pixel; and an extracting unitthat extracts an artifact region, on a basis of attenuation coefficientsof each of the pixels calculated from the pieces of basis material imagedata each of which corresponds to a different one of the plurality ofbasis materials.
 2. The medical image processing apparatus according toclaim 1, further comprising: a correcting unit that corrects theattenuation coefficients at the artifact region, wherein thereconstructing unit generates a monochromatic X-ray image by using thecorrected attenuation coefficients.
 3. The medical image processingapparatus according to claim 2, wherein the correcting unit obtains anenergy level at which each of the attenuation coefficients of theartifact region calculated from pixel values in the pieces of basismaterial image data each of which corresponds to a different one of theplurality of basis materials exhibits a substantially correct value, andthe correcting unit performs a correcting processing by using each ofthe attenuation coefficients of the artifact region at the obtainedenergy level and attenuation coefficients of predetermined materials atthe obtained energy level.
 4. The medical image processing apparatusaccording to claim 3, wherein the correcting unit obtains a valuecalculated in advance as the energy level at which each of theattenuation coefficients of the artifact region exhibits thesubstantially correct value, the attenuation coefficients beingcalculated from the pixel values in the pieces of basis material imagedata each of which corresponds to a different one of the plurality ofbasis materials.
 5. The medical image processing apparatus according toclaim 2, wherein the correcting unit performs a correcting processing byreplacing each of the attenuation coefficients of the artifact regionwith attenuation coefficients of predetermined materials.
 6. The medicalimage processing apparatus according to claim 1, wherein the extractingunit extracts the artifact region from data obtained after a filteringprocessing is performed on the pieces of basis material image data eachof which corresponds to a different one of the plurality of basismaterials.
 7. The medical image processing apparatus according to claim1, wherein the extracting unit extracts the artifact region by comparingattenuation coefficients at two mutually-different energy levels witheach other, within an energy range that does not include absorption edgeenergies.
 8. The medical image processing apparatus according to claim1, wherein the extracting unit extracts such a pixel of which theattenuation coefficients are 0 or smaller as the artifact region.
 9. Themedical image processing apparatus according to claim 1, wherein theextracting unit extracts the artifact region by comparing each of theattenuation coefficients at a predetermined energy level withattenuation coefficients of materials that are set in advance at thepredetermined energy level.
 10. The medical image processing apparatusaccording to claim 1, wherein the extracting unit extracts the artifactregion by comparing a ratio between attenuation coefficients at twomutually-different energy levels with a ratio between attenuationcoefficients of a material that is set in advance at the twomutually-different energy levels.
 11. The medical image processingapparatus according to claim 1, wherein the extracting unit ensures thatsuch a pixel of which pixel values in a mutually same position in thepieces of basis material image data each of which corresponds to adifferent one of the plurality of basis materials fall in apredetermined range is excluded from a target to be extracted as theartifact region.
 12. The medical image processing apparatus according toclaim 1, wherein the projection data is represented by two pieces ofprojection data acquired by using two mutually-different levels of X-raytube voltages.
 13. The medical image processing apparatus according toclaim 2, further comprising: a controlling unit that exercises controlso that the artifact region is displayed while being emphasized withinthe monochromatic X-ray image using the corrected attenuationcoefficients or a monochromatic X-ray image using the attenuationcoefficients before the correction.
 14. An X-ray computed tomographyapparatus comprising: a separating unit that separates projection datainto pieces of line-integrated data each of which corresponds to adifferent one of a plurality of basis materials that are set in advance;a reconstructing unit that reconstructs pieces of basis material imagedata from the pieces of line-integrated data each of which correspondsto a different one of the plurality of basis materials, the pieces ofbasis material image data being configured so that each pixel value ofeach of pixels indicates an abundance ratio of corresponding each of thebasis materials that is present at each of the pixel; and an extractingunit that extracts an artifact region, on a basis of attenuationcoefficients of each of the pixels calculated from the pieces of basismaterial image data each of which corresponds to a different one of theplurality of basis materials.